Separation membrane comprising coating layer and battery using same

ABSTRACT

Disclosed herein is a high thermal resistant polyolefin-based separator including a coating layer containing polyamic acid. 
     Specifically, the separator includes a polyolefin-based substrate film, and a coating layer containing polyamic acid formed on one or both surfaces of the polyolefin-based substrate film, wherein the polyamic acid contains one or more functional groups selected from the group consisting of a sulfone group, a trifluoromethyl group, an alkyl group, and a phenyl ether group. 
     Also, disclosed herein is an electrochemical battery having improved thermal stability by using the separator including a coating layer containing polyamic acid.

TECHNICAL FIELD

The present invention relates to a separator including a coating layer, and an electrochemical battery using the same.

BACKGROUND ART

A separator for an electrochemical battery refers to an intermediate membrane segregating a cathode and an anode from each other in a battery, while continuously maintaining ion conductivity, thereby allowing battery charging and discharging.

Recently, along with weight lightening and miniaturization trend of an electrochemical battery for high portability of an electronic device, a battery is also required to have high power and large capacity in order to be used for an electric car, and the like. Thus, a separator for a battery is required to be thin and have light weight, and at the same time, to have excellent thermal shape stability for production of a high power battery.

Particularly, in case where a polyolefin-based film is used as a substrate film of the separator, the film may melt down even at relatively low temperature, and thus, in order to compensate for such a problem, a study to improve the thermal resistance of the substrate film has proceeded. It is suggested in Korean Patent Registration No. 10-0775310, etc. that a coating layer of a mixture of organic and inorganic materials is formed on one or both surfaces of a substrate film of a separator, in order to improve the thermal resistance of a substrate film.

Meanwhile, an attempt has been made to improve thermal stability of a coating layer, by using an organic binder having excellent thermal resistance such as polyimide as a coating agent component of a separator. However, the organic binder having high thermal resistance does not dissolve in a low boiling point solvent, which later causes a problem of not properly performing a drying process of the solvent after coating a separator. Moreover, this not only decreases air permeability of a separator, but also reduces compatibility with other coating agent components added together so as to make it difficult to be substantially utilized as a coating agent.

Therefore, the development of a separator for a battery having excellent thermal resistance and air permeability, by coating the separator with an organic binder having high thermal resistance and also high solubility in a low boiling point solvent as a coating agent component, is needed.

DISCLOSURE OF INVENTION Technical Problem

An object of the present invention is to provide a separator having excellent thermal resistance and drying processability, by utilizing polyamic acid having excellent solubility in a low boiling point solvent as a coating agent component of a separator.

Another object of the present invention is to provide a separator having a less solvent residual amount in a coating layer so as to have excellent air permeability, and improved thermal resistance.

Another object of the present invention is to provide an electrochemical battery having excellent thermal stability, by using the separator.

Another object of the present invention is to provide a separator maintaining an excellent shutdown function of a polyolefin-based separator, and having improved thermal resistance and air permeability.

Technical Solution

In one general aspect, a polyolefin-based separator includes a coating layer containing polyamic acid.

Specifically, the separator includes a polyolefin-based substrate film, and a coating layer containing polyamic acid formed on one or both surfaces of the polyolefin-based substrate film, wherein the polyamic acid has the structure of following Chemical Formula 1 or 2.

wherein

R₁, R₅ and R₇ are, independently of one another, an unsubstituted or substituted aromatic hydrocarbon having 6 to 30 carbon atoms; an unsubstituted or substituted aliphatic hydrocarbon having 2 to 20 carbon atoms; or an unsubstituted or substituted alicyclic hydrocarbon having 3 to 24 carbon atoms. R₁, R₅ and R₇ may be identical to or different from one another.

In another general aspect, an electrochemical battery includes the separator, a cathode, an anode, and electrolyte.

In another general aspect, a lithium secondary battery includes the separator.

Advantageous Effects

The separator of the present invention maintains a shutdown property as it is, while having improved melt-down temperature and excellent thermal resistance.

Further, the separator of the present invention has a less solvent residual amount in a coating layer of the dried separator, thereby not reducing air permeability, and compensates for the thermal sensitivity of polyolefin, thereby enhancing the thermal resistance of the separator.

The coating composition for a separator of the present invention may have excellent solubility in a low boiling point solvent, and mitigate a drying condition, so that simplification of a process and cost saving are possible.

Further, the separator of the present invention has strong resistance to thermal shrinkage which occurs upon overheating of a battery, and thus, in case of utilizing the separator in a battery, the battery has improved stability and extended life.

Further, the polyamic acid used in the coating composition of the present invention may maintain the dispersibility of inorganic particles in an inorganic dispersion, so as to improve the preparation processability of a coating layer of a mixture of organic and inorganic materials.

The effects according to the present invention are not limited by the illustration above, and various effects are included herein.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing capacity change with the number of cycle of a battery using the separator according to an exemplary embodiment of the present invention.

FIG. 2 is a graph showing the measurement of shutdown temperature of the separator according to Example 1 (Graph B) and Comparative Example 1 (Graph A).

FIG. 3 is a graph showing the result of thermal mechanical analysis (TMA) measurement of the separator according to Example 1 (Graph B) and Comparative Example 1 (Graph A).

FIG. 4 is a graph showing the measurement of a high-rate discharge property (C-rate) of a separator according to Example 1 and Comparative Example 1.

BEST MODE

Hereinafter, the present invention will be described in detail. Since the part not disclosed herein may be sufficiently recognized and inferred by a person skilled in the art to which the invention pertains or in the similar art, a description thereof will be omitted.

The separator according to an exemplary embodiment of the present invention includes a coating layer containing high thermal resistant polyamic acid. Specifically, the separator according to an exemplary embodiment of the present invention includes a polyolefin-based substrate film, and a coating layer containing polyamic acid formed on one or both surfaces of the polyolefin-based substrate film, wherein the polyamic acid has the structure of following Chemical Formula 1 or 2.

wherein

R₁, R₅ and R₇ are, independently of one another, an unsubstituted or substituted aromatic hydrocarbon having 6 to 30 carbon atoms; an unsubstituted or substituted aliphatic hydrocarbon having 2 to 20 carbon atoms; or an unsubstituted or substituted alicyclic hydrocarbon having 3 to 24 carbon atoms. R₁, R₅ and R₇ may be identical to or different from one another.

Specifically, the aromatic hydrocarbon having 6 to 30 carbon atoms may be represented by following Chemical Formula 3:

wherein Ar₁ to Ar₄ are, independently of one another, unsubstituted or substituted arylene having 6 to 15 carbon atoms, and Ar₁ to Ar₄ may be identical to or different from one another; and X₁ to X₃ are, independently of one another, a single bond, O, S, C(═O), S(═O)₂, C(═O)NH, unsubstituted or substituted alkylene having 1 to 10 carbon atoms, specifically alkylene having 1 to 4 carbon atoms, or unsubstituted or substituted silylene. For example, the substituted alkylene or silylene may be mono- or di-substituted by F, OH, CH₃, CF₃ and the like.

m, l and o are, independently of one another, 0 or 1. Specifically, if m is 1, and l and o are 0, X₂ and X₃ are a single bond, and Ar₂ is unsubstituted or substituted trivalent arylene having 6 to 15 carbon atoms, and if m and l are 1, and o is 0, X₃ is a single bond, and Ar₃ is unsubstituted or substituted trivalent arylene having 6 to 15 carbon atoms. If m, l and o are all 0, X₁ to X₃ are a single bond, and Ar₁ is unsubstituted or substituted tetravalent arylene having 6 to 15 carbon atoms, for example, unsubstituted or substituted tetravalent phenylene or naphthylene.

In the Chemical Formula 1 or 2, R₂, R₆ and R₈ are, independently of one another, an unsubstituted or substituted aromatic hydrocarbon having 6 to 30 carbon atoms; an unsubstituted or substituted aliphatic hydrocarbon having 2 to 20 carbon atoms; an unsubstituted or substituted alicyclic hydrocarbon having 3 to 24 carbon atoms; or —R₃—Ar₅—R₄— (wherein R₃ and R₄ are, independently of each other, alkylene having 1 to 5 carbon atoms; and Ar₅ is unsubstituted or substituted arylene having 6 to 15 carbon atoms, and if Ar₅ is substituted, it is mono- to tri-substituted by CH₃, OH, SH or NH₂.). R₂, R₆ and R₈ may be identical to or different from one another.

Specifically, in R₂, R₆ and R₈, the unsubstituted or substituted aromatic hydrocarbon having 6 to 30 carbon atoms may be represented by following Chemical Formula 4:

—(Ar₆)—X₄—(Ar₇)_(p)—X₅—(Ar₈)_(q)—X₆—(Ar₉)_(r)  [Chemical Formula 4]

wherein Ar₆ to Ar₉ are, independently of one another, arylene having 6 to 15 carbon atoms, and Ar₆ to Ar₉ may be identical to or different from one another; and X₄ to X₆ are, independently of one another, a single bond, O, S, C(═O), S(═O)₂, C(═O)NH, unsubstituted or substituted alkylene having 1 to 10 carbon atoms, specifically alkylene having 1 to 4 carbon atoms, or unsubstituted or substituted silylene. For example, the substituted alkylene or silylene may be mono- or di-substituted by F, OH, CH₃, CF₃ and the like.

p, q and r are, independently of one another, 0 or 1. If p is 1, and q and r are 0, X₅ and X₆ are a single bond, and Ar₆ and Ar₇ are, independently of each other, unsubstituted or substituted phenylene or naphthylene. If p and q are 1, and r is 0, X₆ is a single bond. If p, q and r are all 0, X₄ to X₆ are a single bond.

In the Chemical Formula 4, X₄ may be in an ortho-, meta-, or para-position to an amine group (—NH—) of the Chemical Formula 1 or 2. Specifically, it may be in a meta-position, and in case of being in a meta-position to an amine group, its solubility may be improved.

In the Chemical Formula 1, n is an integer of 30 to 10000, an integer of 100 to 1000, an integer of 50 to 500, or an integer of 50 to 300.

In the Chemical Formula 2, x is an integer of 15 to 5000, and y is an integer of 15 to 5000, specifically, x is an integer of 50 to 500, and y is an integer of 50 to 500, and more specifically, x is an integer of 25 to 250, and y is an integer of 25 to 250.

According to an exemplary embodiment of the present invention, in the Chemical Formula 1 or 2, R₁, R₂, R₅, R₆, R₇ and R₈ may be, independently of one another, an unsubstituted or substituted aromatic hydrocarbon having 6 to 18 carbon atoms; an unsubstituted or substituted alicyclic hydrocarbon having 6 to 24 carbon atoms; or an unsubstituted or substituted aliphatic hydrocarbon having 2 to 17 carbon atoms, and in the aromatic hydrocarbon of the Chemical Formula 3 or 4, Ar₁ to Ar₄, or Ar₆ to Ar₉ may be, independently of one another, an unsubstituted or substituted arylene having 4 to 10 carbon atoms, specifically phenylene or naphthylene Herein, in the Chemical Formula 3, m, l and o may be all 0, or m may be 1, and l and o may be 0, and in the Chemical Formula 4, p, q and r may be all 1, or q may be 1, and q and r may be 0

The polyamic acid compound according to another exemplary embodiment of the present invention may be the compound wherein in the aromatic hydrocarbon of the Chemical Formula 3 or 4, Ar₁ to Ar₄ are unsubstituted, and Ar₆ to Ar₉ are, independently of one another, unsubstituted or substituted. If any one or more of Ar₆ to Ar₉ are substituted, they may be substituted by CH₃, CF₃, OH, F or the like, for example, CH₃ or CF₃.

The polyamic acid compound according to another exemplary embodiment of the present invention may be the compound wherein in the Chemical Formula 1 or 2, R₁, R₅ and R₇ are an unsubstituted or substituted phenylene or naphthylene, an unsubstituted or substituted alicyclic hydrocarbon having 4 to 12 carbon atoms, an aliphatic hydrocarbon having 4 to 7 carbon atoms, or two or three unsubstituted or substituted phenyl groups in the form of being connected by a single bond, O, S, C(═O), S(═O)₂, C(═O)NH, unsubstituted or substituted alkylene having 1 to 10 carbon atoms, unsubstituted or substituted silylene, and the like. The two or three unsubstituted or substituted phenyl groups may be, specifically, independently of each other, in the form of being connected by a single bond; C(═O); S(═O)₂; alkylene having 1 to 3 carbon atoms unsubstituted or mono- or di-substituted by CH₃ or CF₃; or silylene unsubstituted or mono- or di-substituted by CH₃ or CF₃. More specifically, the connecting group of the two or three phenyl groups may be a single bond, C(═O), C(CH₃)₂ or C(CF₃)₂. Alternatively, two phenyl groups may be connected by the connecting group. R₂, R₆ and R₈ may be an unsubstituted or substituted alicyclic hydrocarbon having 4 to 12 carbon atoms, an unsubstituted or substituted aliphatic hydrocarbon having 4 to 17 carbon atoms, or two to four unsubstituted or substituted phenyl groups in the form of being connected by a single bond, O, S, C(═O), S(═O)₂, C(═O)NH, unsubstituted or substituted alkylene having 1 to 10 carbon atoms, unsubstituted or substituted silylene, and the like. The two to four unsubstituted or substituted phenyl groups may be, specifically, independently of each other, in the form of being connected by a single bond; S(═O)₂; O; C(═O); alkylene having 1 to 3 carbon atoms unsubstituted or mono- or di-substituted by CH₃ or CF₃; or silylene unsubstituted or mono- or di-substituted by CH₃ or CF₃. More specifically, the connecting group of the two to four phenyl groups may be a single bond, C(═O), S(═O)₂, O, C(CH₃)₂ or C(CF₃)₂. In particular, the connecting group may be C(═O), S(═O)₂ or O. For example, two phenyl groups may be connected by the connecting group.

The polyamic acid compound according to another exemplary embodiment of the present invention may be the compound wherein in the Chemical Formula 1 or 2, R₁ and R₅ are unsubstituted or substituted phenylene or naphthylene, and R₂, R₆, R₇ and R₈ are two to four unsubstituted or substituted phenyl groups in the form of being connected by a single bond, O, S, C(═O), S(═O)₂, C(═O)NH, unsubstituted or substituted alkylene having 1 to 10 carbon atoms, unsubstituted or substituted silylene, or the like.

The polyamic acid having the structure of the Chemical Formula 1 or 2 may represent adequate solubility in the low boiling point solvent (the solvent having a boiling point less than 150° C.). Specifically, if an organic binder component of the separator coating agent has low solubility in the low boiling point solvent, the preparation of the coating agent usable in a general coating method itself may be difficult, and on the contrary, if it has unduly high solubility in the low boiling point solvent, battery safety may be rather decreased due to the risk of dissolving the separator in the electrolyte solution of a battery. The polyamic acid having the structure of the Chemical Formula 1 or 2 has adequately controlled solubility in a low boiling point solvent, and thus, is suitable as a coating composition of a separator. The polyamic acid having the structure of the Chemical Formula 1 or 2 has excellent thermal resistance and solubility in a low boiling point solvent, thereby securing the thermal resistance of the separator with polyamic acid itself without proceeding with imidation separately, and also dispenses with carrying out imidation at high temperature, thereby avoiding the damage of a polyolefin layer which is a lower substrate, and thus, a coating layer may be formed on a polyolefin-based substrate. Further, since its solubility in a low boiling point solvent is excellent, a high temperature drying process under a severe condition for removing a high boiling point solvent is not necessary, and thus, a coating layer may be formed on a polyolefin substrate. Though polyimide has excellent thermal resistance, it does not dissolve in a low boiling point solvent, so that it has not been substantially utilized as a coating agent component of the separator. Specifically, in a separator including a coating layer, if a large amount of solvent remains in the coating layer after drying, the separator will have reduced adhesion, and also decreased air permeability, and thus, not function properly as a separator. Therefore, in case where the separator is intended to be coated by a general coating method (in particular, dip coating), a low boiling point solvent has been used as a coating agent solvent, in order to facilitate drying of the solvent. However, in case of polyimide, since it does not dissolve in the low boiling point solvent, it has been difficult to be utilized in a separator coating layer in the prior art, in spite of its excellent thermal resistance. Thus, the problems in the prior art are intended to be solved in the present invention by introducing polyamic acid which has excellent thermal resistance and easily dissolves in a low boiling point solvent.

The term used herein, ‘aromatic hydrocarbon having 6 to 30 carbon atoms’ includes all the cases where the aromatic hydrocarbon is present alone, two aromatic hydrocarbons are joined to form a condensed ring, or two or more aromatic rings are not joined to each other, but connected by another connecting group. The aromatic hydrocarbon present alone is exemplified by a divalent or tetravalent phenylene group, and two aromatic hydrocarbons joined to each other to form a condensed ring are exemplified by a divalent or tetravalent naphthylene group.

The term used herein, ‘arylene having 6 to 15 carbon atoms’ includes an aromatic hydrocarbon having 6 to 15 carbon atoms wherein the aromatic hydrocarbon is present alone, or two aromatic hydrocarbons are joined to each other to form a condensed ring, and the arylene is exemplified by a phenylene or naphthylene group. The arylene may be di-, tri- or tetravalent, herein.

The term used herein, ‘alicyclic hydrocarbon having 3 to 24 carbon atoms’ refers to a saturated or partially unsaturated hydrocarbon group containing 1 to 3 rings having 3 to 8 carbon atoms, respectively. For example, as the alicyclic hydrocarbon having 6 to 20 carbon atoms, a cyclohexyl, cycloheptyl, cyclohexenyl, or 1,2,3,4-tetrahydronaphthalene group may be mentioned.

The term used herein, ‘aliphatic hydrocarbon having 2 to 20 carbon atoms’ refers to a saturated or partially unsaturated, straight-chained or branched-chained, di- or tetravalent hydrocarbon group. Specifically, it may refer to a saturated, straight-chained or branched-chained, di- or tetravalent hydrocarbon group. As an example of the aliphatic hydrocarbon having 2 to 20 carbon atoms, ethyl, butyl, pentyl, hexyl, 1,1-dimethylbutyl or the like may be used.

In case where the term, ‘unsubstituted or substituted’ is used herein, unless otherwise stated, a group may be unsubstituted or substituted several times by F, OH, SH, CH₃, CF₃, NH₂ or the like. For example, a group may be mono- to tri-substituted by F, OH, SH, CH₃, CF₃, NH₂ or the like.

In case where R₂, R₆ or R₈ is phenylene, or R₂, R₆ or R₈ contains a phenylene group as a part herein, it may be, independently of one another, connected in an ortho-, meta- and para-position.

Specifically, in the Chemical Formulae 1 and 2, R₁, R₅ and R₇ may be selected from the group consisting of following Chemical Formulae A1 to A43.

Specifically, in the Chemical Formulae 1 and 2, R₂, R₆ and R₈ may be selected from the group consisting of following Chemical Formulae B1 to B74.

The polyamic acid compound according to another exemplary embodiment of the present invention may include one or more functional groups selected from the group consisting of a sulfone group, a trifluoromethyl group, an alkyl group and a phenyl ether group. For example, one or more sulfone, trifluoromethyl, alkyl and/or phenyl ether groups may be included in the molecule, and both sulfone and trifluoromethyl groups may be included. If the polyamic acid includes one or more sulfone, trifluoromethyl, alkyl and/or phenyl ether groups, it may have more increased solubility in a low boiling point solvent. Specifically, the polyamic acid represented by any one of following Chemical Formulae 5 to 7 may be used.

The polyamic acid having the structure of the Chemical Formula 5 has excellent solubility in a low boiling point solvent, and specifically it may represent excellent solubility in a solvent composition containing acetone being a low boiling point solvent and N,N-dimethyl acetamide (DMAc) being a high boiling point solvent in a weight ratio of about 9.5:0.5 or less, that is, in a weight ratio of acetone to DMAc of 9.5:0.5 or less. More specifically it may represent excellent solubility in a solvent composition containing acetone and DMAc in a weight ratio of acetone to DMAc of 9:1 or less, or 8.75:1.25 or less.

The polyamic acid having the structure of the Chemical Formula 6 has excellent solubility in a low boiling point solvent, and specifically it may represent excellent solubility in a solvent composition containing acetone being a low boiling point solvent and DMAc being a high boiling point solvent in a weight ratio of about 8:2 or less, that is, in a weight ratio of acetone to DMAc of 8:2 or less. More specifically, it may represent excellent solubility in a solvent composition containing acetone and DMAc in a weight ratio of acetone to DMAc of 7.5:2.5 or less.

The polyamic acid having the structure of the Chemical Formula 7 has excellent solubility in a low boiling point solvent, and specifically it may represent excellent solubility in a solvent composition containing acetone being a low boiling point solvent and DMAc being a high boiling point solvent in a weight ratio of about 7.5:2.5 or less, that is, in a weight ratio of acetone to DMAc of 7.5:2.5 or less. More specifically, it may represent excellent solubility in a solvent composition containing acetone and DMAc in a weight ratio of acetone to DMAc of 6:4 or less.

Further, polyamic acid having the structure wherein an amic acid repeating unit containing a phenyl ether group (x) (hereinafter, referred to as a ‘unit’) and an amic acid unit containing a sulfone group (y) are repeated, or polyamic acid having the structure wherein an amic acid unit containing a sulfone group but not containing a trifluoromethyl group (x) and an amic acid unit containing sulfone and trifluoromethyl groups (y) are repeated, may be used. Specifically, the polyamic acid represented by following Chemical Formula 8 or 9 may be used:

wherein more specifically, a ratio x:y is 5:5 to 1:9. Within the range of the ratio, the polyamic acid solubility in the low boiling point solvent (e.g., acetone) may be increased.

The polyamic acid having the structure of the Chemical Formula 8 may specifically, represent excellent solubility in a solvent composition containing acetone being the low boiling point solvent and DMAc being the high boiling point solvent in a weight ratio of about 8:2 or less. More specifically, it may represent excellent solubility in a solvent composition containing acetone and DMAc in a weight ratio of acetone to DMAc of 7.5:2.5 or less.

wherein a ratio x:y is 9:1 to 7:3. The polyamic acid within the range of the ratio may have not too high solubility in an electrolyte solution and the like, and only sufficiently high solubility in the low boiling point solvent. The polyamic acid having the structure of the Chemical Formula 9 has excellent solubility in the low boiling point solvent, and specifically, may represent excellent solubility in a solvent composition containing acetone being the low boiling point solvent and DMAc being the high boiling point solvent in a weight ratio of about 9.5:0.5 to 5:5.

In the Chemical Formulae 5 to 9, the sulfone group may be a substituent in an ortho-, meta- or para-position to an amine group, for example, in a meta-position. In case where the sulfone group is a substituent in the meta-position, polyamic acid solubility in the low boiling point solvent may be increased.

The polyamic acid according to exemplary embodiments of the present invention may have a weight average molecular weight (Mw) of 50,000 to 100,000. In case of having the molecular weight range, it may have increased solubility in the low boiling point solvent, and improved thermal resistance.

The polyamic acid of the Chemical Formula 1 may be prepared by a method known in the art to react an anhydride containing R₁ with diamine containing R₂. The polyamic acid of the Chemical Formula 2 may be prepared by a method known in the art to react an anhydride containing R₅ with diamine containing R₆, and an anhydride containing R₇ with diamine containing R₈.

The non-limiting example of the anhydride containing R₁, R₅ or R₇ may include pyromellitic dianhydride, 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride, 3,3′,4,4′-benzophenonetetracarboxylic dianhydride, 4,4′-carbonyldiphthalic dianhydride, 1,2,3,4-butanetetracarboxylic dianhydride, 4,4′-oxydiphthalic dianhydride, 3,3′,4,4′-biphenyltetracarboxylic dianhydride, bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride, 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride, 4,4′-isopropylidenediphthalic dianhydride, 1,2,4,5-cyclohexanetetracarboxylic dianhydride, 1,2,3,4-cyclopentanetetracarboxylic dianhydride, or the like.

The non-limiting example of the diamine containing R₂, R₆ or R₈ may include 3,3′-diaminodiphenylsulfone, 4,4′-diaminodiphenyl sulfone, 1,6-hexamethylenediamine, 4,4′-oxydianiline, 4,4′-methylenedianiline, 1,3-phenylenediamine, 1,4-phenylenediamine, 2,2-bis(3-amino-4-methylphenyl)hexafluoropropane, meta-xylenediamine, para-xylenediamine, 3,3′-(hexafluoroisopropylidene)dianiline, 4,4′-(hexafluoroisopropylidene)dianiline), 3-[3-(3-aminophenyl)sulfonylphenyl]sulfonylaniline, 2,2′-bis(trifluoromethyl)benzidine, 1,16-hexadecanediamine, 1,4-cyclohexyldiamine, 3,3′-bis(trifluoromethyl)benzidine, ortho-tolidine, 2,3,5,6-tetramethyl-1,4-phenylenediamine, 2,5-dimethyl-1,4-phenylenediamine, 4,4′-diamino-3,3′-dimethyldiphenylmethane, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, or the like

The polyamic acid having the structure of the Chemical Formula 1 or 2 may represent adequate solubility in the low boiling point solvent (the solvent having a boiling point less than 150° C.)

Specifically, if an organic binder component of the separator coating agent has low solubility in the low boiling point solvent, the preparation of the coating agent usable in a general coating method itself may be difficult, and on the contrary, if it has unduly high solubility in the low boiling point solvent, battery safety may be rather decreased due to the risk of dissolving the separator in the electrolyte solution of a battery.

Thus, in one exemplary embodiment of the present invention, there is provided a coating composition having adequately controlled solubility in the low boiling point by using the polyamic acid having the structure of the Chemical Formula 1 or 2.

The polyamic acid used in the present invention may be contained in 1 to 30% by weight, more specifically 1 to 20% by weight, and for example, 1 to 15% by weight, based on the weight of the coating composition. Within the range, the polyamic acid may sufficiently serve as an organic binder component of the coating agent, and sufficiently impart high thermal resistance to the coating composition.

The low boiling point solvent used in the present invention refers to a solvent having a boiling point less than 150° C. The non-limiting examples of the low boiling point solvent usable in the present invention may include acetone, tetrahydrofuran (THF), and the like. These may be used alone or in a mixture of two or more, and for example, acetone may be used. Since acetone has a significantly low boiling point of about 56.5° C., if it is used as a solvent of a coating agent, the coating layer may be easily dried, so that the air permeability of the separator becomes excellent, and also deterioration of the physical properties due to the residual solvent may be prevented.

According to one embodiment of the present invention, additionally to the low boiling point solvent, a high boiling point solvent may be used together as the solvent of the coating composition.

The high boiling point solvent usable in the present invention refers to a solvent having boiling point of 150° C. or more. The non-limiting example of the high boiling point solvent usable in the present invention may include dimethylformamide (DMF), dimethylsulfoxide (DMSO), dimethylacetamide (DMAc), dimethylcarbonate (DMC), N-methylpyrrolidone (NMP), or the like. These may be used alone or in a mixture of two or more.

As to the contents of the low boiling point solvent and the high boiling point solvent, the weight ratio of the low boiling point solvent (X) to the high boiling point solvent (Y) (X:Y) may be 9.5:0.5 to 5:5, specifically 9.0:1.0 to 5:5, and more specifically 8:2 to 5:5.

In case of adjusting the content ratio of the low boiling point solvent and the high boiling point solvent to the above range, the polyamic acid may be sufficiently dissolved to facilitate the preparation of the coating agent, and the coating layer formed on the substrate film may also be easily dried. That is, the solvent may remain in a small amount in the dried coating layer of the separator (for example, 500 ppm or less), so that the air permeability of the separator will not be decreased.

The total content of the solvent including the low boiling point solvent and the high boiling point solvent may be 20 to 99% by weight, specifically 50 to 95% by weight, and more specifically 70 to 95% by weight, based on the weight of the coating composition. If the solvent is contained within the range, the coating agent may be easily prepared, and the drying process of the coating layer may be performed well.

The inorganic particles used in the present invention are not specifically limited, and any inorganic particles generally used in the art may be used. The non-limiting examples of the inorganic particles usable in the present invention include Al₂O₃, SiO₂, B₂O₃, Ga₂O₃, TiO₂, SnO₂ or the like. These may be used alone or in a mixture of two or more. The inorganic particles used in the present invention may be, for example, Al₂O₃ (alumina).

The size of the inorganic particle used in the present invention is not specifically limited, and the average particle diameter may be 1 to 2,000 nm, or 100 to 1,000 nm. In case of using the inorganic particles within the size range, dispersibility and coating processability of the inorganic particles in a coating solution may be prevented from being reduced, and the thickness of the coating layer may be appropriately adjusted to prevent mechanical property deterioration and increase in electric resistance. Further, the size of the pores produced in the separator may be appropriately adjusted to lower a probability of causing an internal short-circuit upon charging and discharging a battery.

In the preparation of the coating composition, the inorganic particles may be used in the form of an inorganic dispersion being dispersed in an appropriate solvent. The appropriate solvent is not specifically limited, and any solvent generally used in the art may be used. As an appropriate solvent to disperse the inorganic particles, acetone may be used, for example.

The inorganic dispersion may be prepared by a general method without a special limitation, and for example, in a manner of adding Al₂O₃ in a proper amount to acetone, and milling it to be dispersed with a beads mill.

In the preparation of the inorganic dispersion, the content of the inorganic particles may be 10 to 40% by weight, specifically 20 to 30% by weight, based on the weight of the dispersion. If the inorganic particles are contained within the range, their heat dissipation property may be sufficiently exhibited, and the separator coated using them may have effectively suppressed thermal shrinkage.

The content of the inorganic dispersion may be 10 to 70% by weight, specifically 20 to 60% by weight, and more specifically 30 to 50% by weight, based on the weight of the coating composition of the present invention. Within the range, the inorganic particles may be expected to show a sufficient heat dissipation property, and the content of an organic binder is also relatively properly adjusted, and thus, the adhesion of the separator may be secured above a certain level.

According to an exemplary embodiment of the present invention, the coating composition may further include a binder in addition to the polyamic acid. The binder may be one selected from the group consisting of polyvinylidene fluoride (PVdF) homopolymer, polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose and acrylonitrilestyrene-butadiene copolymer, alone or in a mixture thereof. Specifically, polyvinylidene fluoride homopolymer and/or polyvinylidene fluoride-hexafluoropropylene copolymer may be included.

In case of further including polyvinylidene fluoride homopolymer, the viscosity and adhesion of the coating composition may be improved to assist the inorganic particles to be uniformly dispersed, and also, a coating layer having high adhesion may be formed on a substrate film to increase the stability of the separator. The weight average molecular weight of the polyvinylidene fluoride-hexafluoropropylene copolymer usable in the present invention may be 1,000,000 g/mol or less, or for example, 1,000,000 to 1,200,000 g/mol, but not specifically limited thereto. Within the molecular weight range, the adhesion between the coating layer and the polyolefin-based substrate film may be enhanced, so that the heat-sensitive polyolefin-based substrate film may be effectively prevented from shrinking by heat, and additionally, the adhesion between the coating layer and an electrode may also be improved to prevent the short-circuit of a cathode and an anode. Further, in case of using the polyvinylidene fluoride homopolymer within the molecular weight range, the polyvinylidene fluoride homopolymer may dissolve well even with a small amount of DMF, thereby facilitating the drying of the coating layer.

Further, for example, in case of coating the separator with a coating agent further containing polyvinylidene fluoride-hexafluoropropylene copolymer, the electrolyte impregnation property of the separator may be improved to produce a battery having excellent electrical output. The weight average molecular weight of the polyvinylidene fluoride copolymer usable in the present invention may be 800,000 g/mol or more, or for example, 600,000 to 800,000 g/mol, but not specifically limited thereto. In case of using the polyvinylidene fluoride-hexafluoropropylene copolymer within the molecular weight range, a separator having a sufficiently improved electrolyte impregnation property may be prepared, and using the separator, a battery efficiently generating electrical output may be produced.

In the polyvinylidene fluoride-hexafluoropropylene copolymer used in the present invention, each content of the polyvinylidene fluoride and hexafluoropropylene is not specifically limited, but the hexafluoropropylene may be contained in 0.1 to 40% by weight, based on the total weight of the copolymer.

The method of preparing a coated separator according to one exemplary embodiment of the present invention may include applying a coating composition containing polyamic acid, a low boiling point solvent, and a high boiling point solvent on one or both surfaces of a polyolefin-based substrate film, and drying it to form a coating layer.

The preparation of a coating composition containing polyamic acid, a low boiling point solvent, and a high boiling point solvent in the present invention may include mixing 1 to 30% by weight of the polyamic acid, based on the total weight of the coating composition, with 70 to 99% by weight of the low boiling point solvent and the high boiling point solvent, based on the total solvent, and stirring it at 10 to 40° C. for 30 minutes to 5 hours.

The coating composition may further include inorganic particles. Therefore, the preparation of the coating composition may include mixing the polyamic acid, the low boiling point solvent, the high boiling point solvent, and the inorganic particles, and stirring it at 10 to 40° C. for 30 minutes to 5 hours. Herein, the content of the inorganic particles may be 10 to 40% by weight, based on the total weight of the coating composition.

Alternatively, the coating composition may be prepared by dispersing the inorganic particles in a dispersion medium to prepare an inorganic dispersion, and mixing it with a high molecular solution containing the polyamic acid, the low boiling point solvent and the high boiling point solvent. In case of separately preparing the inorganic dispersion as such, the dispersion property and solution stability of the inorganic particles and the polyamic acid may be improved. Therefore, in another embodiment, the coating composition of the present invention may be prepared by mixing the polyamic acid ingredient and the inorganic particles prepared in the state of being dissolved or dispersed in an appropriate solvent, respectively.

For example, the coating composition may be prepared in a manner of preparing solutions of polyamic acid, polyvinylidene fluoride homopolymer and/or polyvinylidene fluoride-hexafluoropropylene copolymer each dissolved in an appropriate solvent, and an inorganic dispersion in which inorganic particles are dispersed, respectively, then mixing them with an appropriate solvent.

After mixing the polyamic acid solution, the inorganic dispersion, and the solvent, the mixture may be sufficiently stirred using a ball mill, a beads mill, a screw mixer, or the like to prepare a coating composition in the form of a mixture.

According to another embodiment of the present invention, there is provided a separator in which the coating composition is coated on one or both surfaces of the polyolefin-based substrate film.

The method to coat the polyolefin-based substrate film with the coating agent is not specifically limited, and any method generally used in the art may be used. The non-limiting example of the coating method may include a dip coating, a die coating, a roll coating, a comma coating method, or the like. These methods may be applied alone or in a combination of two or more. The coating layer of the separator of the present invention may be formed by, for example, a dip coating method.

The coating layer of a mixture of organic and inorganic materials of the present invention may have a thickness of 0.01 to 20 μm, specifically 1 to 15 μm. Within the thickness range, the coating layer may be formed to have an adequate thickness to obtain excellent thermal stability and adhesion, and the separator may be prevented from being unduly thick entirely, thereby suppressing an increase of internal resistance of a battery.

The coating layer may be dried by warm air, hot air or low humidity air, or vacuum dried, or dried by irradiating far-infrared radiation, electron beam or the like in the present invention. In addition, the drying temperature depends on the kind of the solvent, but may be generally 60 to 120° C. The drying time also depends on the kind of the solvent, but may be generally 1 minute to 1 hour. In a specific example, the drying may be carried out at 90 to 120° C. for 1 to 30 minutes, or 1 to 10 minutes In the present invention, the solvent may be effectively removed even under a condition of reduced drying time and lower drying temperature as the above by using the polyamic acid having excellent solubility in the low boiling point solvent as a coating composition component.

After drying, the residual amount of the low boiling point solvent and the high boiling point solvent in the coated separator may be 500 ppm or less. Specifically, the residual amount of the low boiling point solvent and the high boiling point solvent in the coated separator may be 400 ppm or less. For example, after drying, the low boiling point solvent may not remain, and the high boiling point solvent may remain in 500 ppm or less in the coating layer.

It is preferred that the substrate film used in the separator of the present invention is polyolefin-based. The non-limiting example of the polyolefin-based substrate film may include a polyethylene substrate film, a polypropylene substrate film, or the like.

In case of the separator for a secondary battery, a substrate film having a shutdown function is preferred, and the polyolefin-based substrate film used in the separator of the present invention corresponds to the substrate film having an excellent shutdown function.

The polyolefin-based substrate film used in the present invention may be selected from the group consisting of for example, a single polyethylene membrane, a single polypropylene membrane, a double-layered polyethylene/polypropylene membrane, a triple-layered polypropylene/polyethylene/polypropylene membrane, and a triple-layered polyethylene/polypropylene/polyethylene membrane.

The polyolefin-based substrate film may have a thickness of 1 to 40 μm, more specifically 1 to 30 μm, and still specifically 1 to 20 μm. In case of using the substrate film within the thickness range, the prepared separator may have an adequate thickness which is enough to prevent a short-circuit of a cathode and an anode of a battery, but not enough to increase the internal resistance of a battery.

The polyamic acid of the present invention is not imidated in the coating layer, and may be present in the form of polyamic acid.

The residual amount of the organic solvent in the dried coating layer of the separator of the present invention may be 500 ppm or less. The organic solvent residual amount refers to a sum of the residual amounts of the low boiling point solvent and the high boiling point solvent, if both solvents are used.

The solvent residual amount in the dried coating layer of 500 ppm or less in the present invention does not numerically include a value less than zero, and technically refers to a positive (+) value of 0 to 500 ppm.

The dried coating layer of the separator of the present invention refers to a coating layer dried by the drying process at 70 to 120° C., specifically 100 to 120° C. for 1 to 20 minutes, or 1 to 10 minutes, more specifically 1 to 2 minutes, or dried at 10 to 30° C. for 6 to 48 hours, after coating the coating agent on the polyolefin-based substrate film.

In case where the solvent residual amount in the dried coating layer of the separator is 500 ppm or less, the followings may be prevented: a problem caused by an excessive amount of the solvent remaining in the coating layer, that is, an organic binder component not representing sufficient adhesion, and a problem of not effectively suppressing the thermal shrinkage of the substrate film due to reduced adhesion of the coating layer, which accordingly functions as a factor hindering battery performance upon charging/discharging a battery which causes the short-circuit of an electrode when a battery overheats.

The organic solvent remaining in the dried coating layer of the present invention may have a boiling point higher than the melting point of the substrate film of the present invention.

When measuring a temperature at which a separator breaks by pulling the separator coated with the polyamic acid with a force of 0.005 N at a heating rate of 5° C./rain (see ASTM E 831), the breaking temperature may be 180° C. or more. Within the range, the separator does not shrink well even at high temperature, so that the stability of the separator and a battery may be improved. Therefore, in another exemplary embodiment of the present invention, there is provided a separator including a polyolefin-based substrate film; and a coating layer containing polyamic acid formed on one or both surfaces of the substrate film, wherein the separator has a breaking temperature of 180° C. or more under a condition of 0.005 N, and a heating rate of 5° C./min.

The separator coated with the polyamic acid of the present invention may have, after being left at 200° C. for 1 hour, thermal shrinkage in a machine direction (MD) or a transverse direction (TD) of 20% or less, specifically 10% or less, more specifically 5% or less, respectively. The thermal shrinkage may be 5% or less, for example. Within the range, the short-circuit of an electrode may be effectively prevented to improve the stability of a battery.

The separator coated with the polyamic acid of the present invention may have, after being left at 150° C. for 1 hour, thermal shrinkage in a machine direction (MD) or a transverse direction (TD) of 15% or less, specifically 13% or less, more specifically 10% or less, respectively. The thermal shrinkage may be 5% or less, for example. Within the range, the short-circuit of an electrode may be effectively prevented to improve the stability of a battery.

The method to measure the thermal shrinkage of the separator is not specifically limited, and any method generally used in the art may be used.

The non-limiting example of the method of measuring the thermal shrinkage of the separator is as follows: After a prepared separator is cut into a size of about 5 cm (MD) x about 5 cm (TD), and stored in a chamber at 200° C. for 1 hour, the shrinkage in MD and TD directions of the separator is measured to calculate the thermal shrinkage.

The thermal shrinkage at 150° C. may be measured in the same manner as the above method, except that the chamber at 200° C. is replaced with the chamber at 150° C.

The separator of the present invention may have thermal resistance temperature of 200° C. or more. If the separator has the thermal resistance temperature of 200° C. or more, an electrode short-circuit phenomenon by heat may be effectively suppressed, thereby manufacturing a battery having high thermal safety. ‘Thermal resistance temperature’ used herein, refers to temperature to which when the separator is exposed for 10 minutes, the shrinkage of the separator in MD/TD directions is less than 5%.

According to another embodiment of the present invention, there is provided an electrochemical battery including a porous polyolefin-based separator containing the coating layer of a mixture of organic and inorganic materials, and a cathode and an anode, and filled with electrolyte.

The kind of the electrochemical battery is not specifically limited, and may be any kind known to the art.

The electrochemical battery of the present invention may be, specifically, a lithium secondary battery such as a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary batter, a lithium ion polymer secondary battery, or the like.

The method of manufacturing the electrochemical battery of the present invention is not specifically limited, and any method generally used in the art may be used.

The non-limiting example of the method of manufacturing the electrochemical battery is as follows: The battery may be manufactured in a manner of disposing the polyolefin-based separator including the coating layer of a mixture of organic and inorganic materials between a cathode and an anode of the battery, and then filling it with an electrolyte solution.

The electrode forming the electrochemical battery of the present invention may be prepared in the form of binding an electrode active material to an electrode current collector by a method generally used in the art.

The cathode active material of the electrode active material used in the present invention is not specifically limited, and any cathode active material generally used in the art may be used.

The non-limiting example of the cathode active material may include lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, a lithium complex oxide combining those oxides, or the like.

The anode active material of the electrode active material used in the present invention is not specifically limited, and any anode active material generally used in the art may be used.

The non-limiting example of the anode active material may include a lithium adsorption material such as a lithium metal or lithium alloy, carbon, petroleum coke, activated carbon, graphite, or other carbons.

The electrode current collector used in the present invention is not specifically limited, and any electrode current collector generally used in the art may be used.

The non-limiting example of the materials of the cathode current collector of the electrode current collector may include foil prepared from aluminum, nickel or a combination thereof, or the like.

The non-limiting example of the materials of the anode current collector of the electrode current collector may include foil prepared from copper, gold, nickel, a copper alloy or a combination thereof, or the like.

The electrolyte solution used in the present invention is not specifically limited, and any electrolyte for an electrochemical battery generally used in the art may be used.

The electrolyte solution may be one wherein a salt having a structure such as A⁺B⁻ is dissolved or dissociated in an organic solvent.

The non-limiting example of A⁺ may include a cation selected from the group consisting of an alkali metal cation such as Li⁺, Na⁺ or K⁺, or a combination thereof.

The non-limiting example of B⁻ may include an anion selected from the group consisting of an anion such as PF₆ ⁻, BF₄ ⁻, Cl⁻, Br⁻, I⁻, ClO₄ ⁻, AsF₆ ⁻, CH₃CO₂ ⁻, CF₃SO₃ ⁻, N(CF₃SO₂)₂ ⁻ or C(CF₂SO₂)₃ ⁻, or a combination thereof.

The non-limiting example of the organic solvent may include propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropylcarbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethylcarbonate (EMC), γ-butyrolactone, or the like. These may be used alone or in a mixture of two or more.

Hereinafter, the present invention is described in more detail, by describing the following Examples, Comparative Examples, and Experimental Examples. However, the Examples, Comparative Examples, and Experimental Examples are only illustrative of the present invention, and the disclosure of the present invention is not construed as being limited thereto.

Examples 1 to 20 Preparation of Separator Having a Coating Layer Containing Polyamic Acid Formed Thereon Example 1 (1) Preparation of Coating Composition

0.5 mol of 3,3′-diamino diphenyl sulfone (DDS), and N,N-dimethylacetamide (DMAc) were placed in a four-neck flask equipped with a stirrer, a temperature controlling device, a nitrogen gas injection device, and a condenser while passing nitrogen therethrough, and dissolved with stirring. Next, 0.5 mol of pyromellitic dianhydride (PMDA) in a solid state was added to the solution, and the solution was vigorously stirred. Herein, the solid content by mass ratio was 20% by weight, and the reaction proceeded for 24 hours while maintaining the temperature less than 25° C. to prepare a polyamic acid solution.

In order to prepare an inorganic dispersion, 25% by weight of Al₂O₃ (LS235, Nippon Light Metal Company, Ltd.) was added to acetone (Daejung Chemical & Metals Co., Ltd.), and milled at 25° C. for 3 hours using a beads mill to be dispersed, thereby preparing an inorganic dispersion.

The thus prepared, polyamic acid solution and inorganic dispersion were mixed in a weight ratio of polyamic acid solution:inorganic dispersion:N,N-dimethylacetamide (DMAc):acetone=1:4.8:0.8:3.4, and stirred at 25° C. for 2 hours with a power mixer to prepare a coating composition.

(2) Preparation of Separator Having Coating Layer Containing Polyamic Acid Formed Thereon

The above prepared coating composition was coated on both surfaces of the polyethylene substrate film having a thickness of 12 μm in a manner of dip coating, and then dried at room temperature for 24 hours to prepare a separator.

Example 2

The separator was prepared by the same method as in above Example 1, except that 1,6-hexamethylenediamine was placed in the four-neck flask instead of 3,3′-diaminodiphenylsulfone, and the mixing was carried out in a weight ratio of the prepared polyamic acid solution:N,N-dimethylacetamide (DMAc):inorganic dispersion:acetone=0.9:1.5:4.1:3.5, in the preparation of the coating composition.

Example 3

The separator was prepared by the same method as in above Example 1, except that 4,4′-oxydianiline was placed in the four-neck flask instead of 3,3′-diaminodiphenylsulfone, the solid content of the polyamic acid solution by mass ratio was 10% by weight, and the mixing was carried out in a weight ratio of the prepared polyamic acid solution:N,N-imethylacetamide (DMAc):inorganic dispersion:acetone=0.9:2.8:4.1:2.2, in the preparation of the coating composition.

Example 4

The separator was prepared by the same method as in above Example 1, except that 0.2 mol of 4,4′-oxydianiline and 0.3 mol of 3,3′-diaminodiphenylsulfone were placed in the four-neck flask instead of 0.5 mol of 3,3′-diaminodiphenylsulfone, and the mixing was carried out in a weight ratio of the prepared polyamic acid solution:N,N-dimethylacetamide (DMAc):inorganic dispersion: acetone=0.9:1.5:4.1:3.5, in the preparation of the coating composition.

Example 5

The separator was prepared by the same method as in above Example 1, except that 4,4′-diaminodiphenylsulfone was placed in the four-neck flask instead of 3,3-diaminodiphenylsulfone, in the preparation of the coating composition.

Example 6

The separator was prepared by the same method as in above Example 1, except that 0.375 mol of pyromellitic dianhydride and 0.125 mol of 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride were used instead of 0.5 mol of pyromellitic dianhydride, and the mixing was carried out in a weight ratio of the prepared polyamic acid solution:N,N-dimethylacetamide (DMAc):inorganic dispersion:acetone=0.9:0.2:4.1:4.7, in the preparation of the coating composition.

Example 7

The separator was prepared by the same method as in above Example 1, except that polyvinylidene fluoride homopolymer (hereinafter, referred to as ‘PVdF homopolymer’) solution was further included, and the mixing was carried out in a weight ratio of the prepared polyamic acid solution:PVdF homopolymer solution: N,N-dimethylacetamide (DMAc):inorganic dispersion: acetone=0.5:0.7:0.7:4.1:4.0, in the preparation of the coating composition.

The PVdF homopolymer solution was prepared by adding 10% by weight of PVdF homopolymer (5130, Solvay) to DMF (Daejung Chemical & Metals Co., Ltd.), and stirring it at 25° C. for 4 hours using a stirrer.

Example 8

The separator was prepared by the same method as in above Example 1, except that polyvinylidene fluoride-hexafluoropropylene copolymer (hereinafter, referred to as ‘PVdF-HFP copolymer’) solution was further included, and the mixing was carried out in a weight ratio of the prepared polyamic acid solution:PVdF-HFP copolymer solution:N,N-dimethylacetamide (DMAc):inorganic dispersion: acetone=0.5:0.7:1.4:4.1:3.3, in the preparation of the coating composition.

The PVdF-HFP copolymer solution was prepared by adding 10% by weight of PVdF-HFP copolymer having a weight average molecular weight of 700,000 g/mol (21216, Solvay) to acetone (Daejung Chemical & Metals Co., Ltd.), and stirring it at 25° C. for 4 hours using a stirrer.

Example 9

The separator was prepared by the same method as in above Example 1, except that 0.45 mol of pyromellitic dianhydride and 0.05 mol of 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride were used instead of 0.5 mol of pyromellitic dianhydride, and the mixing was carried out in a weight ratio of the prepared polyamic acid solution:N,N-dimethylacetamide (DMAc):inorganic dispersion:acetone=0.9:0.3:4.1:4.7, in the preparation of the coating composition.

Example 10

The separator was prepared by the same method as in above Example 1, except that 0.35 mol of pyromellitic dianhydride and 0.15 mol of 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride were used instead of 0.5 mol of pyromellitic dianhydride, and the mixing was carried out in a weight ratio of the prepared polyamic acid solution:N,N-dimethylacetamide (DMAc):inorganic dispersion:acetone=0.9:0.1:4.1:4.7, in the preparation of the coating composition.

Example 11

The separator was prepared by the same method as in above Example 1, except that 0.3 mol of pyromellitic dianhydride and 0.2 mol of 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride were used instead of 0.5 mol of pyromellitic dianhydride, and the mixing was carried out in a weight ratio of the prepared polyamic acid solution:inorganic dispersion:acetone=0.9:4.1:4.7, in the preparation of the coating composition.

Example 12

The separator was prepared by the same method as in above Example 1, except that 1,2,3,4-Butanetetracarboxylic dianhydride was used instead of pyromellitic dianhydride, in the preparation of the coating agent.

Example 13

The separator was prepared by the same method as in above Example 1, except that 1,2,4,5-cyclohexanetetracarboxylic dianhydride was used instead of pyromellitic dianhydride, in the preparation of the coating agent.

Example 14

The separator was prepared by the same method as in above Example 1, except that 4,4′-carbonyldiphthalic dianhydride was used instead of pyromellitic dianhydride, in the preparation of the coating agent.

Example 15

The separator was prepared by the same method as in above Example 1, except that 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride, and 1,16-hexadecanediamine were used instead of pyromellitic dianhydride, and 3,3′-diamino diphenyl sulfone, respectively, in the preparation of the coating agent.

Example 16

The separator was prepared by the same method as in above Example 1, except that 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride, and 1,4-cyclohexyldiamine were used instead of pyromellitic dianhydride, and 3,3′-diamino diphenyl sulfone, respectively, in the preparation of the coating agent.

Example 17

The separator was prepared by the same method as in above Example 6, except that 4,4′-diamino diphenyl sulfone was used instead of 3,3′-diamino diphenyl sulfone, in the preparation of the coating agent.

Example 18

The separator was prepared by the same method as in above Example 6, except that 1,2,4,5-cyclohexanetetracarboxylic dianhydride was used instead of pyromellitic dianhydride, in the preparation of the coating agent.

Example 19

The separator was prepared by the same method as in above Example 6, except that 4,4′-isopropylidenediphthalic dianhydride was used instead of 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride, in the preparation of the coating agent.

Example 20

The separator was prepared by the same method as in above Example 1, except that 3-[3-(3-aminophenyl)sulfonylphenyl]sulfonylaniline was used instead of, and 0.375 mol of pyromellitic dianhydride and 0.125 mol of 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride were used instead of 0.5 mol of pyromellitic dianhydride, in the preparation of the coating agent.

The solvent ratios used in the above Examples are shown in the following Table 1.

TABLE 1 Acetone DMAc Example 1 8.15 1.85 Example 2 7.3 2.7 Example 3 6 4 Example 4 7.3 2.7 Example 5 8.15 1.85 Example 6 8.94 1.06 Example 9 8.85 1.15 Example 10 9.04 0.96 Example 11 9.15 0.85 Example 12 8.15 1.85 Example 13 8.15 1.85 Example 14 8.15 1.85 Example 15 8.15 1.85 Example 16 8.15 1.85 Example 17 8.94 1.06 Example 18 8.94 1.06 Example 19 8.94 1.06 Example 20 8.15 1.85

Comparative Examples 1 and 2 Preparation of Separator Having a Coating Layer not Containing Polyamic Acid Formed Thereon Comparative Example 1

A coating agent prepared by mixing in a compositional ratio of the PVdF-HFP copolymer solution of Example 8: the inorganic dispersion of Example 1: acetone=2:4.8:3.2, and stirring it at 25° C. for 2 hours with a power mixer, was coated on both surfaces of a polyethylene substrate film having a thickness of 14 μm in a manner of dip coating, and dried to prepare the separator.

Comparative Example 2

The separator was prepared in the same method as in Comparative Example 1, except that the PVdF homopolymer solution of Example 7 was used instead of the PVdF-HFP copolymer solution.

TABLE 2 Molecular Polyamic acid weight Ex- ample 1

40,000~60,000 Ex- ample 2

60,000~80,000 Ex- ample 3

 90,000~110,000 Ex- ample 4

 80,000~100,000 x:y = 4:6 Ex- ample 5

60,000~80,000 Ex- ample 6

60,000~80,000 x:y = 7.5:2.5 Ex- ample 7

40,000~60,000 Ex- ample 8

40,000~60,000 Ex- ample 9

60,000~80,000 x:y = 9:1 Ex- ample 10

60,000~80,000 x:y = 7:3 Ex- ample 11

60,000~80,000 x:y = 6:4 Ex- ample 12

40,000~60,000 Ex- ample 13

50,000~70,000 Ex- ample 14

60,000~80,000 Ex- ample 15

40,000~60,000 Ex- ample 16

60,000~80,000 Ex- ample 17

70,000~90,000 x:y = 7.5:2.5 Ex- ample 18

70,000~90,000 x:y = 7.5:2.5 Ex- amples 19

60,000~80,000 x:y = 7.5:2.5 Ex- ample 20

70,000~90,000 x:y = 7.5:2.5

Experimental Example 1 Solubility in Low Boiling Point and High Boiling Point Solvents According to Structure of Polyamic Acid

In order to evaluate the solubility of the polyamic acids prepared according to the above Examples each having different structure in a low boiling point solvent and a high boiling point solvent, the ratios of the low boiling point solvent and the high boiling point solvent at which the solvents are maintained in the most clear state when each of the polyamic acid is dissolved therein, were measured. Acetone (Daejung Chemical & Metals Co., Ltd) was used as the low boiling point solvent, and N,N-dimethylacetamide (DMAc) was used as the high boiling point solvent.

As to the low boiling point solvent and the high boiling point solvent of each of the polyamic acids, the results of measuring the acetone: DMAc ratios at which the most clear state is maintained, are shown in the following Table 3:

TABLE 3 Examples Acetone:DMAc 1 8.75:1.25 2 7.5:2.5 3 6:4 4 6.66:3.34 5 8.33:1.67 6 9.09:0.91 9  8.88:.1.12 10 9.44:0.56 11 Dissolved in an electrolyte solution 12 9:1 13  8.88:.1.12 14 9:1 15 9.33:0.67 16 9.23:0.77 17 9.33:0.67 18 9.41:0.59 19 9.41:0.59 20 9.47:0.53

As shown in the above Table 3, it was confirmed that the polyamic acids prepared in above Examples 1 to 20 have excellent solubility in acetone which is the low boiling point solvent, and thus, are suitable for being utilized in the coating composition of the separator.

In particular, the polyamic acid having the highest solubility in acetone was those prepared according to Examples 6, 9, 10 and 11, and when comparing these polyamic acids, it was confirmed that as the ratio of 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride increases, the solubility of the polyamic acid in acetone increases.

However, it was observed that the polyamic acid of Example 11 dissolved also in an electrolyte solution of a battery, as a result of its excessively increased solubility in the low boiling point solvent.

Experimental Example 2 Measurement of Thickness and Coated Weight of Coating Layer

The following method was carried out in order to measure the thicknesses and the coated weights of the coating layers of the separators prepared in above Examples 1 to 20, and Comparative Examples 1 and 2.

First, the thickness of each coating layer was measured using a SEM cross section image of each coating layer and micro calipers. Then, each coating layer was cut into a size of 10 cm (MD)×20 cm (TD) and its weight was measured with an electronic scale to calculate the coated weight. The results of measuring the thicknesses and the coated weights are shown in the following Table 5.

Experimental Example 3 Measurement of Thermal Shrinkage of Separator

The following method was carried out in order to measure the thermal shrinkage of the separators prepared in above Examples 1 to 20, and Comparative Examples 1 and 2.

Each separator prepared according to the above Examples and Comparative Examples was cut into a size of 5 cm (MD)×5 cm (TD), thereby producing a total of seven samples. Each of the samples was stored in chambers at 150° C. and 200° C. for 1 hour, and then shrinkage amounts in MD and TD directions of each sample were measured to calculate the thermal shrinkage. The results of measuring the thermal shrinkage are shown in the following Table 5.

Experimental Example 4 Measurement of Air Permeability

The air permeability of the separators prepared in above Examples 1 to 20, and Comparative Examples 1 and 2 was measured by determining the time to pass 100 cc of air through the separator using EG01-55-1MR (Asahi Seiko Co., Ltd.).

Experimental Example 5 Electrolyte Solution Wettability of Separator

The following method was carried out in order to measure the electrolyte solution wettability of the separators prepared in above Examples 1 to 20, and Comparative Examples 1 and 2.

Each separator prepared according to the above Examples and Comparative Examples was cut into a square of 3 cm (MD)×3 cm (TD), thereby producing a total of seven samples. After each sample was floated on the surface of an electrolyte solution in a beaker, time to be completely soaked by the electrolyte solution was measured.

The time to be soaked by the electrolyte solution is shown in the following Table 5.

Experimental Example 6 Measurement of DMAc Solvent Residual Amount in Coating Layer of Separator

In order to measure the organic solvent residual amounts in the coating layers of the dried separators prepared in above Examples 1 to 20, and Comparative Examples 1 and 2, gas chromatography (HP-6890) was performed under conditions described in the following Table 4, and the results are shown in Table 5.

TABLE 4 Parameters Conditions Column Front: HP-INNOWax (length 30M, ID 0.53 mm, film thickness 1.00 μm) Back: HP-1 (length 30M, ID 0.53 mm, film thickness 0.88 μm) Temperature 40° C. (4 min.) →20° C./min → 250° C. (4 min.) and time Flow rate 10 mL/min Injector S/SL Injector Split ratio 5:1 Detector FID Injection 1 

volume Injector 200° C. temperature

TABLE 5 Thick- Coated nesses of weights of Solvent Electrolyte Air Coating coating residual solution permeability Thermal shrinkage of separator (%) layer layer amount wettability (Sec/100 150° C., 1 hour 200° C., 1 hour (μm) (g/m²) (ppm) (sec) cc) TD MD TD MD Example 1 6.1 7.3 120 30 550 0.5 0.6 0.9 1.0 Example 2 6.6 7.5 115 40 650 0.5 0.6 0.9 1.0 Example 3 6.9 7.7 310 45 790 0.3 0.6 0.8 0.9 Example 4 6.8 7.6 180 35 650 0.4 0.5 0.9 1.0 Example 5 6.4 7.6 125 34 600 0.5 0.5 1.0 1.0 Example 6 6.2 7.2 55 31 403 0.5 0.7 1.1 1.3 Example 7 8.1 8.4 80 55 426 1.1 1.1 2.0 2.1 Example 8 7.8 8.1 85 38 438 1.4 1.3 2.5 3.0 Example 9 6.9 7.8 55 32 380 0.5 0.7 0.9 1.0 Example 10 6.3 7.5 56 32 380 0.5 0.6 1.0 1.1 Example 11 6.0 7.0 55 30 350 0.6 0.7 1.1 1.2 Example 12 6.2 7.3 115 35 360 0.7 0.5 0.9 0.8 Example 13 6.7 7.5 120 37 340 0.6 0.7 0.9 1.0 Example 14 6.5 7.4 120 40 330 0.5 0.8 0.8 1.1 Example 15 6.1 7.1 120 41 290 0.6 1.0 0.9 1.2 Example 16 6.3 7.3 115 40 295 0.5 0.7 1.0 1.3 Example 17 6.0 7.0 60 38 270 1.3 1.4 2.0 2.5 Example 18 6.0 7.0 55 37 265 1.4 1.5 2.5 3.5 Example 19 6.0 7.0 60 32 280 0.6 0.8 1.2 1.3 Example 20 6.0 7.0 95 40 295 1.0 1.2 1.9 2.1 Comparative 8.8 8.5 — 65 414 21.5 26.0 — — Example 1 Comparative 9.0 9.0 — >300 455 10.0 12.5 — — Example 2

As shown in the above Table 5, it is confirmed that the separators of Examples 1 to 20 containing polyamic acid in the coating layers have less thermal shrinkage amounts, and thus, have excellent thermal resistance, as compared with those of Comparative Examples 1 and 2 not containing polyamic acid.

Further, when comparing the separators of Examples 6, 9, 10 and 11, as the ratio of 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride increases, the solubility of the polyamic acid in acetone increases (see Table 1), and thus, it was confirmed that as the solvent is easily volatilized, drying performance is improved, and as a result, the solvent residual amount of the coating layer is small, so that the air permeability becomes excellent.

Meanwhile, after a lithium secondary battery was manufactured using the separator according to Example 1, battery capacity change by the use of the battery was observed, and as a result, it was confirmed that a battery capacity was hardly changed even after about 350 cycles (FIG. 1).

Accordingly, it is considered that in case of utilizing the separator of the present invention in an electrochemical battery, thermal stability of the battery may be improved, such that a battery life is extended over a long term.

Experimental Example 7 Measurement of Shutdown Function of Separator

In order to measure the shutdown function of each separator according to above Example 1 and Comparative Example 1, the impedance of the separators prepared in Example 1 and Comparative Example 1 was measured at 1 kHz and a heating rate of 10° C./min using 3522-50 LCR HiTester (HIOKI) (FIG. 2).

It is shown in FIG. 2 that the shutdown temperatures of Example 1 and Comparative Example 1 were similar, and when comparing it with the results of Experimental Example 3, it was confirmed that the separator of Example 1 having a coating layer containing polyamic acid formed thereon had improved thermal resistance, while still maintaining the shutdown function excellent, and thus, the high temperature safety of the separator may be secured.

Experimental Example 8 Measurement of TMA (Thermal Mechanical Analysis)

In order to measure TMA of each separator according to above Examples and Comparative Examples, the temperature at which each separator breaks was measured using TA Instruments (TMA Q400) by pulling each separator with a force of 0.005 N at a heating rate of 5° C./min (see ASTM E 831) (FIG. 3). The results are shown in the following Table 6.

It is shown in FIG. 3 that the separator of Comparative Example 1 was observed to have pore shrinkage in the vicinity of shutdown temperature (about 130° C.), and represented a breaking property at about 150° C. or more. On the contrary, the separator of Example 1 hardly showed shrinkage up to 191° C., due to the high temperature safety of the coating layer containing polyamic acid. Further, according to Table 6, the separator including the polyamic acid coating layer showed a breaking temperature of 180° C. or more.

TABLE 6 Breaking temperature (° C.) Example 1 191 Example 2 190 Example 3 188 Example 4 187 Example 5 190 Example 6 188 Example 7 176 Example 8 179 Example 9 188 Example 10 190 Example 11 189 Example 12 188 Example 13 187 Example 14 190 Example 15 190 Example 16 189 Example 17 187 Example 18 188 Example 19 191 Example 20 188 Comparative 145 Example 1 Comparative 144 Example 2

Experimental Example 9 Measurement of High-Rate Discharge Property (C-Rate)

The high-rate discharge property of each separator according to Example 1 and Comparative Example 1 was measured (FIG. 4). The high-rate discharge property was measured by observing a capacity change rate by determining the capacities of a battery having a capacity of 850 mAh, wherein the capacity was determined after charging for 2 hours and then discharging for 5 hours (0.2 C), after charging for 2 hours and then discharging for 2 hours (0.5 C), after charging for 2 hours and then discharging for hour (1 C), and after charging for 2 hours and then discharging for 30 minutes (2 C), using a charge/discharge tester (TOSCAT-3600, TOYO Systems Co., Ltd.).

It is shown in FIG. 4 that the separator of Example 1 represented a much excellent high-rate discharge property as compared with the separator of Comparative Example 1. As the high-rate discharge property is superior, the output property of a battery is improved, and thus, the separator of Example 1 may significantly contribute to improved safety of a battery, and also development of a higher output battery, due to an excellent high temperature property. 

1. A separator, comprising: a polyolefin-containing substrate film; and a coating layer including polyamic acid on at least one surface of the substrate film, wherein the polyamic acid is represented by the following Chemical Formula 1 or 2:

wherein: R₁, R₅ and R₇ are, independently of one another, an unsubstituted or substituted aromatic hydrocarbon having 6 to 30 carbon atoms; an unsubstituted or substituted aliphatic hydrocarbon having 2 to 20 carbon atoms; or an unsubstituted or substituted alicyclic hydrocarbon having 3 to 24 carbon atoms; R₂, R₆ and R₈ are, independently of one another, an unsubstituted or substituted aromatic hydrocarbon having 6 to 30 carbon atoms; an unsubstituted or substituted aliphatic hydrocarbon having 2 to 20 carbon atoms; an unsubstituted or substituted alicyclic hydrocarbon having 3 to 24 carbon atoms; —R₃—Ar₅—R₄—, wherein R₃ and R₄ are, independently of each other, an alkylene having 1 to 5 carbon atoms; and Ar₅ is an arylene having 6 to 15 carbon atoms that is unsubstituted or mono- to tri-substituted by CH₃, OH, SH or NH₂; n is an integer of 30 to 10000; and x is an integer of 15 to 5000, and y is an integer of 15 to
 5000. 2. The separator as claimed in claim 1, wherein R₁, R₅ and R₇ are, independently of one another, an aromatic hydrocarbon represented by the following Chemical Formula 3, and R₂, R₆ and R₈ are, independently of one another, an aromatic hydrocarbon represented by the following Chemical Formula 4:

wherein, in Chemical Formula 3: Ar₁ to Ar₄ are, independently of one another, an unsubstituted or substituted arylene having 6 to 15 carbon atoms; X₁ to X₃ are, independently of one another, a single bond, O, S, C(═O), S(═O)₂, C(═O)NH, an unsubstituted or substituted alkylene having 1 to 10 carbon atoms, or an unsubstituted or substituted silylene; and m, l and o are, independently of one another, 0 or 1; with the provisos that: if m is 1, and l and o are 0, X₂ and X₃ are a single bond, and Ar₂ is an unsubstituted or substituted trivalent arylene having 6 to 15 carbon atoms; if m and l are 1, and o is 0, X₃ is a single bond, and Ar₃ is an unsubstituted or substituted trivalent arylene having 6 to 15 carbon atoms; and if m, l and o are all 0, X₁ to X₃ are a single bond, and Ar₁ is an unsubstituted or substituted tetravalent arylene having 6 to 15 carbon atoms; —(Ar₆)—X₄—(Ar₇)_(p)—X₅—(Ar₈)_(q)—X₆—(Ar₉)_(r)  [Chemical Formula 4] wherein, in Chemical Formula 4: Ar₆ to Ar₉ are, independently of one another, an unsubstituted or substituted arylene having 6 to 15 carbon atoms; X₄ to X₆ are, independently of one another, a single bond, O, S, C(═O), S(═O)₂, C(═O)NH, an unsubstituted or substituted alkylene having 1 to 10 carbon atoms, or an unsubstituted or substituted silylene; and p, q and r are, independently of one another, 0 or 1; with the provisos that: if p is 1, and q and r are 0, X₅ and X₆ are a single bond; if p and q are 1, and r is 0, X₆ is a single bond; and if p, q and r are all 0, X₄ to X₆ are a single bond.
 3. The separator as claimed in claim 1, wherein the polyamic acid contains a sulfone group.
 4. The separator as claimed in claim 3, wherein the sulfone group is a meta-aryl sulfone group.
 5. The separator as claimed in claim 1, wherein the polyamic acid contains a sulfone group and a trifluoromethyl group.
 6. The separator as claimed in claim 1, wherein the polyamic acid includes a first repeating unit containing a phenyl ether group and a second repeating unit containing a sulfone group, and a ratio of the first repeating unit to the second repeating unit is 5:5 to 1:9.
 7. The separator as claimed in claim 1, wherein the polyamic acid includes a first repeating unit containing a sulfone group and a second repeating unit containing a trifluoromethyl group, and a ratio of the first repeating unit to the second repeating unit is 9:1 to 7:3.
 8. The separator as claimed in claim 1, wherein the polyamic acid has a structure of following Chemical Formula 9:

wherein a ratio of repeating units x to y is 9:1 to 7:3.
 9. The separator as claimed in claim 8, wherein one or more aryl sulfone groups in the repeating unit x or y of the Chemical Formula 9 are a meta-aryl sulfone group.
 10. The separator as claimed in claim 1, wherein the coating layer contains inorganic particles.
 11. The separator as claimed in claim 10, wherein the inorganic particles include one or more of Al₂O₃, SiO₂, B₂O₃, Ga₂O₃, TiO₂ or SnO₂.
 12. The separator as claimed in claim 1, wherein the separator has a breaking temperature of 180° C. or more under a condition of 0.005 N, and a heating rate of 5° C./min.
 13. An electrochemical battery, comprising: a cathode, an anode, the separator as claimed in claim 1, and an electrolyte.
 14. The electrochemical battery as claimed in claim 13, wherein the electrochemical battery is a lithium secondary battery. 